A study of the threshold characteristics of quantum-dot (QD) laser diodes shows how inhomogeneous broadening and p-doping influence the QD laser's temperature dependence of threshold T0. The analysis includes the additional parameters of homogeneous broadening, quantum state populations, and threshold gain. The results show that while the source of negative T0 can occur due to different effects, the transparency current plays a critical role in both undoped and p-doped QD lasers. Experimental trends of negative T0 and their dependence on p-doping are replicated in the calculated results. Inhomogeneous broadening is found to play a lesser role to the transparency current in setting T0. Homogeneous broadening is most important for uniform QDs with thermally isolated ground-state transitions.